Technical Field
The present disclosure relates to wireless communication between a transponder and a reader, such as a high frequency communication between a RFID transponder and a reader operating at 13,56 MHz, RFID transponders using active load modulation (ALM) for communicating with the reader and to the synchronization between the reader carrier clock and the ALM carrier clock.
Description of the Related Art
A transponder may be a passive one, e.g., performing load modulation of the magnetic field generated by the reader.
A transponder may be an active one.
When a transponder is an active one, e.g., using active load modulation (ALM) for transmitting information to the reader, the transponder generates a magnetic field which simulates load modulation of the reading device field performed by a passive transponder.
ALM may be used in case a signal generated by passive load modulation is not strong enough to be detected by an RFID interrogator device or reader. This may be the case when transponder's antenna is small or located in a challenging environment.
ALM systems actively transmit ALM carrier bursts, the frequency of the ALM carrier being the carrier frequency of the reader. This means that each burst of the ALM carrier starts with the same phase difference to the carrier signal emitted by the reader. Unchanged phase also implies that the two frequencies are ideally identical. Maximum phase drift of transponder reply signal comparing to reader carrier signal inside complete transponder reply frame is specified to be 30° in a draft amendment to the ISO/IEC 14443 standard.
Constant phase difference between the ALM carrier and reader carrier can be achieved for example in the following way.
Once the ALM transmission starts (a complete transmission sequence is called a frame and comprises a series of carrier bursts separated by gaps) a frequency source in the transponder is used to generate ALM carrier clock. The frequency source is occasionally corrected during the gaps in the frame when actual carrier bursts are not emitted. During these gaps only the reader carrier signal is present (on the transponder antenna) so the reference frequency is available to re-adjust the ALM carrier frequency source. Such synchronization between the reader carrier frequency and the ALM carrier frequency is performed within each transmitted frame and is called In-Frame Synchronization (IFS).
An example of a device performing an IFS is disclosed in EP 272 7255 B1.
More precisely a phase locked loop (PLL) with a voltage controlled oscillator (VCO), which generates a clock with a frequency equal to a reader carrier frequency, is used to generate the ALM clock. Outside of transponder reply frame, when the carrier clock is constantly available at PLL input, PLL is put in lock mode. In this lock mode, feedback is closed and the PLL is locked to digitized reader carrier signal.
During the transponder reply frame, when ALM transmission takes place, the PLL is put in hold mode.
In the hold mode, PLL feedback is opened and the VCO continues to run with a frequency which was previously established. The free-running frequency of the VCO is defined by a charge which is stored in a loop filter. A frequency difference between the ALM carrier clock generated by the VCO and reader carrier clock results in a phase drift of the ALM carrier clock compared to the reader carrier clock.
This frequency difference is caused by several sources.
A first source is the difference of frequency generated by the PLL system in which the VCO is induced and the input carrier clock at the moment before the PLL is put on hold.
A second source is charge injection of a switch which puts the PLL in hold mode (charge injected by the switch changes the voltage on the VCO control pin, which results in a change of VCO frequency).
A third effect is leakage on the VCO control pin and the loop filter (loop filter input is in high impedance state, a voltage on this input is defined by stored charge, the charge is modified by leakage current of the electronic elements).
The phase drift generated by sources of frequency difference mentioned above has to be corrected before it drifts above the specified maximum. This is done by closing the PLL feedback loop at appropriate moments inside a transponder reply frame, where the ALM transmission does not take place and the clock extracted from the reader carrier signal is restored on the PLL input.
However, ALM carrier bursts produce at the transponder antenna, signal oscillations after each ALM carrier bursts generation. And, such oscillations may disturb the synchronization between the reader carrier frequency and the ALM carrier frequency.
A solution to this problem has been disclosed in WO 2015/003870 A1.
More precisely, in this document, the ALM carrier bursts are generated from a subcarrier modulation by a Binary Phase Shift Keying (BPSK) encoding.
The synchronization is performed in the gap between two transmission bursts. However, due to the higher amplitude during the transmission bursts period and the oscillation properties of the antenna, the signal amplitude on the antenna of the transponder decays slowly without application of specific measures. And, without such specific measures, the decay of the signal on the inductive capacity antenna lasts too long and does not allow enough time for resynchronization.
Accordingly, the solution disclosed in WO 2015/003870 A1 to shorten this decay comprises performing a controlled damping of the oscillation by specific damping means which quickly stops the oscillation built by ALM transmission.
However, such a damping system increases complexity of the ALM transponder.
The European patent application filed under number 17169020, proposed to perform said in-frame synchronization of transponder when some phase change of BPSK code takes place, without performing any controlled damping through a dedicated damping system.